There is a demand for size, weight and power-consumption reductions in electric actuators used in vehicles such as automobiles. To meet such a demand, such an electric actuator includes a highly efficient, high-performance brushless motor using a rare earth magnet. The rare earth magnet is known to be irreversibly demagnetized when the surrounding temperature rises beyond a predetermined value, which immediately and seriously degrades the performance of the motor. To avoid such demagnetization, a motor for driving a vehicle such as an electric vehicle (EV) or a hybrid vehicle (HEV) is provided with a temperature sensor, and controlled on the basis of its status monitored with the temperature sensor.
Unlike this vehicle drive motor, however, providing any temperature sensor might not be allowed in a motor for driving vehicle accessories such, for example, as a motor driving an electric oil pump for transmission's idle reduction (I/R) since there are layout and cost constraints in such a motor.
To address this, Patent Document 1 discloses the following method for controlling a motor for driving an electric oil pump. In the method, first, the temperature of a magnet in the motor at a time point, while the motor is driving the electric oil pump, is estimated from the current oil temperature with magnet temperature rises by heat generation (calculated with the torque and the rotation speed of the motor) of the motor being added on. Then, the output torque of the motor is limited and the minimum rotational speed is lowered in accordance with the estimated magnet temperature.
Meanwhile, the technique disclosed in Patent Document 2 addresses the above by the following method. First, at a time point when the rotation speed of the motor becomes equal to or more than a predetermined value while the motor is being driven, the magnetic force (magnet temperature) in the motor is estimated from the electromotive force and the rotation speed of the motor. Then, when the magnet is determined to be demagnetized on the basis of the estimated magnetic force, the timing of supplying the current to the motor is changed.